docker exec Command Reference

Run a command inside a running container.

docker exec executes a new process inside an already-running container. In CI it drives database migrations, runs test suites against a service container, or inspects state for debugging. Drop -t in pipelines to avoid TTY errors.

Common flags

-i, --interactive - keep stdin open
-t, --tty - allocate a TTY (omit in non-interactive CI)
-e KEY=value - set an environment variable for the exec
-w, --workdir - working directory for the command
-u, --user - user to run as inside the container
-d, --detach - run the command in the background

Example

shell

docker exec -e RAILS_ENV=test app-web bin/rails db:migrate
docker exec -w /app -u node app-web npm run lint

In CI

Use docker exec (without -t) to run migrations or tests inside a service container started by docker run -d or compose up -d. Adding -t in a pipeline triggers "the input device is not a TTY"; use -i alone or neither.

Key takeaways

exec runs in an existing container; run starts a new one.
Omit -t in CI to avoid TTY allocation errors.
-w and -u control the working directory and user for the command.